This invention relates to a coincidence circuit, and more particularly to such a circuit for use with a.c. input signals in protective relaying applications.
As a result of advancements in solid state technology, static relays have been developed which employ a phase comparator method of measurement. Exemplary circuits for phase comparator static relays are disclosed in U.S. Pat. No. 4,034,269, Wilkinson, entitled "Protective Relay Circuits", issued July 5, 1977. The patent is assigned to the assignee of the present invention and is hereby incorporated by reference in the present application.
In the phase comparator method of measurement, the relay operates by measuring the angle between two or more phasors derived within the relay. Briefly, this is accomplished by the use of an a.c. coincidence circuit in combination with a timer circuit. For example, in the case of the phase distance relay of FIGS. 6 and 7 of previously referenced U.S. Pat. No. 4,034,269, a coincidence detector 286 develops an output signal when phase coincidence exists between a polarizing signal (V.sub.AB) and an operating signal (I.sub.AB Z.sub.R -V.sub.AB). A timer 288 then measures the duration of the phase coincidence and develops an output signal when phase coincidence occurs for a predetermined minimum time equal to the relay operate time. It is to be appreciated that, for faults within the intended reach of the relay, the phase coincidence is greater than 1/4 cycle. Thus, for 60 cycle protective relaying applications, where 1/4 cycle equals 4.167 ms, the timer is ordinarily set at 4.167 ms.
Although a.c. coincidence circuits are presently available for the above-described protective relaying application, such coincidence circuits present several difficulties. One such difficulty is that available coincidence circuits for protective relaying applications generally require square wave input signals of considerable signal strength. For example, such a-c input signals generally must be greater than 71/2 volts rms. This means that the a.c. input signals whose coincidence is to be determined must generally be processed with additional circuitry before coupling to the coincidence circuit. In protective relaying applications, the a.c. input signals are generally in sinusoidal form so the additional circuitry typically is in the form of a squaring amplifier. This additional processing circuitry is undesirable for cost and space reasons. Another difficulty with available coincidence circuits for protective relaying applications is that there is no provision for convenient threshold level adjustments of phase coincidence. The above-cited difficulties are made more serious for those protective relaying applications in which the phase coincidence of more than two a.c. input signals is to be determined.
Accordingly, it is a general object of this invention to provide a coincidence circuit for protective relaying applications which operates directly from a.c. input signals of relatively low signal strength.
It is another object of this invention to provide such a coincidence circuit which provides simple threshold level adjustments.
It is another object of this invention to provide such a coincidence circuit for use with at least three a.c. input signals.
It is another object of this invention to provide such a coincidence circuit for protective relaying applications which includes switching means for determining the same phase or opposite phase coincidence of the a.c. input signals.